In the November/ December 1993 issue of BrewingTechniques, Ron Barchet described the importance of removing hot trub. In this issue, he discusses cold trub and its implications for brewers. Although brewers' opinions vary as to cold trub's ultimate impact on finished beer, certain facts are generally agreed upon. The presentation in this article is descriptive, not prescriptive.

Cold trub consists of proteins, protein-polyphenol complexes, and carbohydrates (Table I). The protein component of cold trub consists of by-products from the breakdown of hordein (prolamin) and globulin. These by-products are soluble in hot wort but precipitate as the wort cools. Approximately 15-25% of these proteins bind to polyphenols, forming protein-polyphenol complexes. Higher molecular weight carbohydrates, called beta-glucans, make up 20-30% of cold trub (2).

As Table I shows, the composition of cold trub produced varies with the percent difference between fine and coarse grist extract and the degree of malt modification. Highly modified malts, for example, yield a higher percentage of polyphenols in cold trub than do less-modified malts. Undermodified malts yield more protein and relatively fewer polyphenols (3).

Undermodified malts yield fewer polyphenols and more beta-glucans (Table I). Wort produced using finely milled malt contains a large quantity of cold trub because the finely ground husks allow greater polyphenol extraction. On the other hand, more-intensive mashing programs, such as double and triple decoction, degrade proteins more extensively and yield less cold trub than do infusion brews, in which more high molecular weight proteins pass through to the chilled wort (Table III).

Boiling the wort reduces the quantity of cold trub, yet adding hops, which contain polyphenols, increases cold trub. Hops and hop products that contain or contribute fewer polyphenols yield less cold trub (4). Furthermore, adding hops late in the boil increases the amount of cold trub formed. Cold trub precipitates best between 41 and 32 °F (5). Thus, to minimize cold trub in a hop accented beer, one should have a lagering capability of 29-32 °F.

REMOVING COLD TRUB: HOW IMPORTANT?

Opinions vary as to the exact effects of removing cold trub. The way cold trub removal affects beer depends on yeast strain, the number of yeast generations used, the method and amount of removal, and the overall brewery characteristics.

It is widely believed that removing all cold trub not only has no benefit, but actually might slow fermentation and harm the finished beer, reportedly giving it an onion-like flavor. Stroh Brewing Co. reported slower fermentation, higher acetate ester levels, and lower yeast growth and viability after removing all cold trub from test batches (6). Further experiments showed another effect of the complete elimination of trub: the absence of nucleation sites during fermentation resulted in a supersaturation of carbon dioxide in the wort; high levels dissolved carbon dioxide inhibit fermentation. Stroh's work revealed the importance of having at least some wort solids present to act as carbon dioxide nucleation sites.

Removing at least some cold trub, however, has been shown to improve yeast viability and the quality of finished beers (3). Studies performed in Germany (where wort and beer are all-malt) have shown that partial removal of cold trub is beneficial to the stability of packaged beer. Flotation accomplishes this by at least partially removing proteins, polyphenols, carbohydrates and heavy metals, such as copper ions and iron complexes, which can catalyze oxidation reactions (7). Taste tests have also shown a preference for beers that were brewed with yeast harvested from beer from which trub had been removed by flotation.

Yeast growth and fermentation in high-gravity worts may benefit from leaving cold trub in the wort, which will lower the carbon dioxide in the fermenting wort, thus repressing the carbon dioxide's inhibitory effect on fermentation.

The cost of extra equipment and labor and the increased risk of contamination are valid arguments for not removing cold trub.

METHODS OF COLD TRUB REMOVAL

Settling tank: The simplest method of removing cold trub, and one used by most home brewers, is the settling tank. Because hot trub also settles, settling tanks can be used for removing both hot and cold trub.

Settling tanks can be used with pitched or unpitched wort. If your lag time to fermentation is <12 h, it is best to remove the cold trub before pitching. For longer lag times, you can pitch first and remove the trub just before active fermentation begins.

Removing cold trub from pitched wort. Move chilled, aerated and pitched wort to a tank, allow it to settle for 12-16 h, and then rack to another tank, leaving the sediment behind. Racking should be done before the active phase of fermentation begins because the natural circulation of carbon dioxide in fermentation will rouse the settled trub. Yeast strains that are highly flocculent may also settle during the lag phase and become mixed with trub. If using such a strain, overpitch by the amount that settles out in the cold trub. As much as 30% of the total cold trub settles out of pitched wort after a 12-h rest (5).

Removing cold trub from unpitched wort. Again, the wort stands for 12-16 h. It is then moved to another vessel, aerated, and pitched with yeast. Sedimentation of unpitched worts is more complete than that of pitched worts (~50%) (5).

Because wild yeast and bacteria are a concern and must be avoided, care must be taken to ensure that the settling tank is sanitary, particularly if the wort is unpitched. Infection will not only affect the finished beer but contaminate the yeast as well.

Shallow vessels rather than cylindroconical tanks make the best settling tanks. Deeper vessels require more time but can benefit from an addition of 10-20 g/hL (12-24 g/bbl) coarse diatomaceous earth. As much as 70% of total cold trub precipitates when using diatomaceous earth (3).

Flotation: Flotation is becoming more common as a method of removing cold trub. In flotation, the cooled wort is saturated with sterile air. As the air bubbles make their way to the top of the tank, they carry cold trub with them. After 2-3 h, a brown, compact head forms at the top of the wort. Wort is then removed from the bottom of the tank, leaving the cold trub behind. Even more cold trub (50-65% of total cold trub) can be removed if wort stands 6-8 h before being racked (3). Brewery characteristics will determine optimal standing time.

It is important not to allow the cold trub cake to break and fall into the clean wort, which will occur if the wort stands too long or if transfer is not halted before the cake is broken. In-line sight glasses are helpful in judging when to stop transfer. Quickly cooling wort and aerating with turbulence will promote good flocculation of cold trub, making it easier to remove before fermentation (3).

The amount of air needed for flotation is ~40-60 L/hL (47-70 L/bbl), depending on bubble size (2). Air is usually injected through a ceramic or metal carbonating stone (pore size ~5 µm) (3) on the cold side of the heat exchanger.

Other methods: Other methods of cold trub removal include centrifugation and diatomaceous earth filtration. A centrifuge used to remove hot break can also be used to eliminate cold break. However, because particle size is smaller and viscosity is higher in cold wort, the plate spacing and angular velocity must be adjusted. Approximately 50-60% of total cold trub can be removed using centrifugation (3).

Diatomaceous earth filtration is the most effective means of removing cold trub. Typically, coarse diatomaceous earth is used, and the best results occur at low temperatures. On average, diatomaceous earth filtration will remove 75-85% of total cold trub. Higher rates (95%) of removal are possible with fine diatomaceous earth and 32 °F wort (3). Diatomaceous earth filtration risks removing too much cold trub, resulting in poor yeast growth and fermentation. Wort must be fully aerated after it is centrifuged or filtered with diatomaceous earth.

CONCLUSION

Although some controversy surrounds this subject, I believe that removing some cold trub, like removing hot trub, might provide benefits to the finished beer from certain breweries. Unlike hot trub removal, however, complete removal of cold trub is neither possible nor desirable. The effects of cold trub removal are subtle and may not be noticeable for several generations. It may be detrimental to some beers, especially those made from high-gravity worts. Cold trub removal might promote quick, clean fermentations by enhancing yeast viability. It also can extend the shelf life of beer by removing oxidation catalysts and haze-forming constituents.

Most American breweries no longer remove cold trub. Some European breweries, notably German, do practice some form of cold trub removal, usually flotation. It is obvious that brewers' opinions vary, a fact that probably reflects the subtle nature of the effects of cold trub removal as well as the dynamics of each brewery and its ingredients.

As to which cold trub removal procedure is best, it is difficult to generalize. I recommend trying different methods on various beers and noting the differences in the resulting beer and in future generations of beer fermented with yeast harvested from worts from which cold trub has been removed. Pale, fine beers such as pilsners are more likely to showcase improvements than are robust, darker styles.